linux-kernel - Re: [RFC PATCH v1 07/20] gpio: Add output event generation method to GPIOLIB and PMC Driver

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Message-ID: <20210917072755.d4ynxkp4scxrk6rq@pengutronix.de>
Date:   Fri, 17 Sep 2021 09:27:55 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     "D, Lakshmi Sowjanya" <lakshmi.sowjanya.d@...el.com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        "open list:PWM SUBSYSTEM" <linux-pwm@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mark Gross <mgross@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Saha, Tamal" <tamal.saha@...el.com>, bala.senthil@...el.com,
        Dipen Patel <dipenp@...dia.com>
Subject: Re: [RFC PATCH v1 07/20] gpio: Add output event generation method to
 GPIOLIB and PMC Driver

Hello,

On Thu, Sep 16, 2021 at 11:42:04PM +0200, Linus Walleij wrote:
> Hi Lakshmi,
> 
> On Tue, Aug 24, 2021 at 6:48 PM <lakshmi.sowjanya.d@...el.com> wrote:
> 
> > From: Lakshmi Sowjanya D <lakshmi.sowjanya.d@...el.com>
> >
> > Intel Timed I/O hardware supports output scheduled in hardware. Enable
> > this functionality using GPIOlib
> >
> > Adds GPIOlib generate_output() hook into the driver. The driver is
> > supplied with a timestamp in terms of realtime system clock (the same
> > used for input timestamping). The driver must know how to translate this
> > into a timebase meaningful for the hardware.
> >
> > Adds userspace write() interface. Output can be selected using the line
> > event create ioctl. The write() interface takes a single timestamp
> > event request parameter. An output edge rising or falling is generated
> > for each event request.
> >
> > The user application supplies a trigger time in terms of the realtime
> > clock the driver converts this into the corresponding ART clock value
> > that is used to 'arm' the output.
> >
> > Work around device quirk that doesn't allow the output to be explicitly
> > set. Instead, count the output edges and insert an additional edge as
> > needed to reset the output to zero.
> >
> > Co-developed-by: Christopher Hall <christopher.s.hall@...el.com>
> > Signed-off-by: Christopher Hall <christopher.s.hall@...el.com>
> > Signed-off-by: Tamal Saha <tamal.saha@...el.com>
> > Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@...el.com>
> > Reviewed-by: Mark Gross <mgross@...ux.intel.com>
> 
> So this is some street organ machine that generates sequences
> with determined timing between positive and negative edges
> right?
> 
> I can't see how this hardware is different from a PWM, or well
> I do to some extent, you can control the period of several
> subsequent waves, but that is really just an elaborate version
> of PWM in my book.

From looking in the patch I think this is more versatile than the PWM
framework abstracts. I wonder if there is a usecase for the
functionality that cannot be expressed using pwm_apply_state?!

I remember we had approaches before that implemented repeating patterns
(something like: active for 5ms, inactive for 10 ms, active for 30 ms,
inactive for 10 ms, repeat) and limiting the number of periods
(something like: .duty_cycle = 5ms, .period = 20ms, after 5 periods go
into inactive state). These were considered to be too special to be
abstracted in drivers/pwm.

> It seems to me that this part of the functionality belongs in the
> PWM subsystem which already has interfaces for similar
> things, and you should probably extend PWM to handle
> random waveforms rather than trying to shoehorn this
> into the GPIO subsystem.

I agree that GPIO is a worse candidate than PWM to abstract that. But
I'm not convinced (yet?) that it's a good idea to extend PWM
accordingly.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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